1. Field of the Invention
The present invention relates to voltage reference circuits for providing a reference voltage, and more particularly to voltage reference circuits having compensation for variations in operating temperature.
2. History of the Prior Art
In electronic design, stable voltage references are needed for a wide variety of analog and digital applications. Such applications include voltage regulators, current supplies for ECL logic, and so forth. For lower voltages, however, and also where stability over a large temperature range is required, providing a precision reference voltage poses considerable problems. In particular, a voltage reference having good stability over a wide temperature range, such as the standard military specification temperature range of -55.degree. C. to +125.degree. C., is very difficult to achieve in a commercially practical implementation.
One conventional approach to providing a voltage reference has been to use temperature compensated zener diodes. However, the disadvantages of that approach have led to another approach in which the band-gap voltage of silicon is employed as an internal reference to provide a regulated output voltage. In this latter approach, two scaled voltages having positive and negative temperature coefficients are added together. The positive temperature coefficient is provided by the difference between the base-emitter voltages of two bipolar transistors operating at different emitter current densities. Because the two transistors are operated at different current densities, a differential in emitter-base voltages of the two transistors is created. The negative temperature coefficient may be taken from the base-emitter junction of a third transistor. The resulting 3-transistor band-gap circuit or cell achieves offsetting temperature coefficients.
An example of a voltage reference circuit which employs a band-gap reference circuit or cell in conjunction with circuitry for providing a temperature compensating current is provided by U.S. Pat. No. 5,053,640 of Yum, which patent issued Oct. 1, 1991 and is commonly assigned with the present application. The '640 patent describes a voltage reference circuit which employs a 2-transistor band-gap cell in combination with circuitry for providing a temperature compensating current. A current controlled current source maintains equal and constant currents through a pair of transistors in the band-gap cell. The current density at the emitter of one of the transistors establishes a positive temperature coefficient voltage drop across a resistor coupled to the emitter of the other transistor, where the difference in base-emitter voltages appears. This produces a voltage across a second resistor which is proportional to absolute temperature and which provides a positive temperature coefficient that offsets the negative coefficient of the base-emitter voltage of the first transistor. From the second resistor, a voltage proportional to absolute temperature is applied to a circuit for generating a temperature compensating current. The temperature compensating current is provided to a voltage divider network which provides the reference voltage. The introduction of the temperature compensating current at the divider network offsets the parabolic temperature dependence of the reference voltage. As a result, the temperature characteristic of the voltage reference circuit is relatively flat or constant over a relatively broad range of operating temperatures.
In the voltage reference circuit described in the '640 patent, the temperature compensating current is applied to alter a ratio provided by the voltage divider network, as described above. As a result, the voltage drop varies and this changes the reference voltage accordingly. A different approach which has been used is to apply the temperature compensating current to alter the band-gap voltage itself. In one example of a voltage reference circuit of this type, the temperature compensating current is applied to the junction between a serial pair of resistors serially coupled between a negative output terminal and the emitter of one of the transistors of the band-gap reference circuit or cell. The temperature compensating current changes the voltage across the resistor coupled to the negative output terminal, which in turn changes a band-gap voltage appearing across one of a pair of resistors serially coupled between the negative output terminal and a positive output terminal. The negative and positive output terminals provide the desired reference voltage therebetween. Because the temperature compensating current must change the voltage across one of the pair of resistors serially coupled between the negative output terminal and the emitter of one of the transistors in the band-gap reference circuit, the temperature compensation provided by a given amount of temperature compensating current is limited. The amount of temperature compensation can be increased by increasing the size of the resistor coupled to the negative output terminal or by increasing the temperature compensating current, but these solutions involve tradeoffs. It would therefore be desirable to provide a voltage reference circuit of this general type, in which the temperature compensation provided by a given amount of temperature compensating current is optimized.
It is therefore an object of the invention to provide an improved voltage reference circuit. A more specific object is the provision of a voltage reference circuit in which the breakpoint compensation of the band-gap reference circuit is optimized.